Carbon Fiber used in Fiber Reinforced Plastic (FRP)

Understanding the fundamentals of carbon fiber is very beneficial to
engineering FRP structure reinforcement and strengthening applications.
These are very high strength fibers used in fiber reinforced plastics
(FRPs). Carbon FRP, or CFRP, has a significant role in structure
strengthening applications and will no doubt become more popular in its
use as original, internal reinforcement.

This page describes the source of carbon based fiber, an explanation of
the different qualities of fiber manufactured, the mechanical properties
of each fiber type, and the difference between "carbon" and "graphite"
fibers.

Fiber Source

Carbon fiber is produced from a petroleum pitch, rayon, or
polyacrylonitrile (PAN) precursor filaments. Of these, PAN-based fiber
is the predominate precursor used in CFRP for structural reinforcement
and strengthening due to its resulting high quality and strength
characteristics.

The process of making PAN-based fibers starts
with acrylonitrile, a monomer, which is a yellowish liquid with the
chemical formula C3H3N. It’s basically a vinyl with a nitrile
attached to it. Acrylonitrile is copolymerized with another monomer to
form the polymer polyacrylonitrile (PAN) resin.

The PAN
precursor is spun and stretched into acrylic fibers, orienting the
polymer molecules and then oxidized at 200 °C to 300 °C. The fibers are
then carbonized by heating in an inert gas atmosphere under tension to
temperatures between 1,500 °C and 2,000 °C for high strength fibers and
up to 2,500 °C for high modulus fibers. After carbonization, the
filaments are sized and given a surface treatment to allow them to bond
well to matrix resin in a future FRP. Manufacture of PAN-based fibers results in a fiber that is about 94% carbon atoms.

Hexagonal Pattern of Carbon Chains

Carbon fibers are essentially ribbons of carbon atoms oriented in the
long direction of the fiber and through the manufacturing process
become folded in a crimped or hairpin fashion. If you could see carbon
filaments on a molecular scale, they might look like a length of
scrunched up chicken wire. Chicken wire fencing has a similar hexagonal
pattern that carbon atoms form when bonded together in long ribbons.

This spring I’ll probably have to buy a short chicken wire fence to keep the rabbits out of my daughter’s vegetable garden.

If I were to take a long length of that 20-inch (0.5 m) tall fence
and fold it in half length-wise, then again so it’s the same length but
it’s only one-quarter the height, then folded and scrunched it in a
random way to as thin a size as, say, my wrist, that would be similar to the
structure of a carbon filament 50,000 times larger than the real
thing, except that the filaments use many, many layers (ribbons) of
carbon that are all folded and scrunched together.

The folds do
not allow the ribbons of carbon to slide past each other. These
interconnected and folded carbon ribbons give carbon-based fibers their great
strength.

Physical and Mechanical Properties

Physical Description

Carbon fiber made from PAN precursor have a diameter of 5 to 7 microns, about 10 times thinner than a human hair.

The filaments are produced and wound onto bobbins in groups called tows.
“Tow” is a synonym in the textile industry to “roving” which is simply
a parallel group of untwisted filaments. Carbon fiber tows generally
contain between 3,000 and 60,000 filaments each. The number of
filaments in a tow is reported in the industry by its thousand, or K
value. For example, a 12K carbon tow has 12,000 carbon filaments in a
tow.

The desired characteristics of the end product determines which size
carbon tow to use. A product, say a carbon textile, weaved with 24K
tows will have a lighter weight and less tensile strength than a product
using 48K tows. CFRPs used in structural reinforcement generally use
large tows (greater than 40K) because they are generally applied to
consistent and uniform surfaces (as externally applied) or formed in
straight sections (as pultruded reinforcing bar or plate).

Strength and Stiffness

Carbon fiber has the widest range of strength and stiffness than all
other fibers used in structural FRPs. Carbon filament is grouped into
categories based on its modulus of elasticity. The classification names
are: low, standard, intermediate, high, and ultra-high modulus. There is no testing standard related to carbon filament classification therefor the elastic
modulus values for each class vary from manufacturer to manufacturer.

Even though the class names are generalized, they divide up how and
where these fibers are generally used. Described below are the
values obtained from the Japan Carbon Fiber Manufacturers Association
(JCMA).

Low
Modulus carbon fiber is generally not used for structural FRPs. Their
low elastic modulus and low tensile strength is not worth the higher
price of carbon versus other fibers such as glass.

Note that the tensile strengths of High and Ultra-High Modulus fibers
are equal to or lower than the strengths of Standard and Intermediate
Modulus fibers. Generally speaking, for FRP reinforcement and strengthening of civil
structures it’s the tensile strength that is desired.

Standard and
Intermediate Modulus fiber is generally known as commercial or
industrial grade fiber. They have high strength and do not have a very
high price tag of High Modulus fibers. The popular carbon fibers used in CFRPs for structure reinforcement
and strengthening generally fall into a range of 32,000 to 35,000 ksi
(220 to 240 GPa) and tensile strengths between 550 and 725 ksi (3,800 to
5,000 MPa). These fibers give the highest value in terms of tensile
strength versus cost.

Carbon Fiber Cost

High and ultra-high stiffness fibers are made for the aerospace industry. They are expensive
and used in specialized applications such as airfoils. At $900 USD per pound ($1,980 USD per kg) for Ultra-High
Mod fiber compared to $25 USD per pound ($55 USD per kg) for Standard
Modulus fiber, it's not economical for the civil infrastructure industry. (2010 prices).

2013 Price Update! Costs of standard modulus carbon fiber have gotten as low as approximately $10 USD per pound ($22 USD per kg).

Graphite Fiber, A Special Note

There is a common misnomer regarding carbon and graphite fibers. Although
both are very similar in composition, they do not have the same
molecular properties. It is incorrect to use graphite fiber as an equal
synonym for carbon fiber.

Graphite is a naturally occurring
soft mineral. Graphite fiber can be manufactured in a process similar
to carbon fiber. In the manufacture of graphite fiber, the precursor is
heated to temperatures up to 2,800 °C. Graphitization, as this process
is called, breaks down the interlocking nature of the carbon ribbons
and forms parallel sheets, the most stable form of graphite.

Graphitization
creates an even more brittle (very, very high elastic modulus) fiber
that has low tensile strength. Remember, pure graphite in a flaky or
powdery form is used as a lubricant (When I was in Cub Scouts, my dad
showed my how to lube the wheels of my Pinewood Derby car with
graphite). The molecular carbon sheets or platelets that form graphite
are very slippery and slide past one another easily.

Graphite
has been used as a term relating to carbon fiber in the sports industry.
This means that when you’re at the store checking out the latest golf
clubs that have “graphite shafts” or fly rods made with “graphite
blanks” you’re really looking at products made with carbon filaments.

Prince Engineering Can Help!

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assistance to engineers and projects in the U.S., Haiti, and Nicaragua.

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